1. Field of the Invention
The present invention relates to load control devices for providing variable power to alternating-current (AC) loads, for example, motor loads, such as AC fan motors. More particularly, the invention relates to quiet fan speed control, specifically for controlling of the speed of a ceiling-mounted cooling fan quickly while minimizing the generation of acoustic noise.
2. Description of the Related Art
A problem with known techniques for controlling the speed of fan motors is that some of the methods have produced substantial amounts of acoustic noise in the fan motor and the fan speed control, i.e., the control device operating the fan motor. FIG. 1A shows a prior art variable fan speed control 10. The fan speed control 10 is coupled between an AC power source 16 and a fan motor 18. The fan motor 18 is modeled as an inductor in series with a resistor. The fan motor of a typical ceiling fan has large resistive component, which causes the fan motor 18 to appear mostly resistive to the fan speed control 10.
A controllably conductive switch 12, typically comprising a bidirectional semiconductor switch, such as a triac, is controlled by a control circuit 14 to change the phase angle at which the triac begins conducting each half-cycle of the AC power source, thereby providing variable speed control. As well known to those skilled in the art, by controlling the phase angle at which the triac begins conducting (i.e., the conduction time of the triac each half-cycle of the AC power source), the amount of power delivered to the fan motor 18 and thus the speed of the fan motor, can be controlled.
A problem with the prior art fan speed control 10 is that when a fan motor 18 is controlled by the phase angle technique, mechanical and acoustic noises are generated in the fan motor, which can be annoying and distracting. FIG. 1B shows the waveforms of the AC input voltage 19A, the motor voltage 19B applied to the fan motor 18, and the motor current 19C through the fan motor. As can be observed from the waveforms, the motor voltage 19B has large discontinuities, and thus harmonics, which cause noise and vibration to be generated in the fan motor 18. The harmonics in the motor voltage 19B delivered to the fan motor 18 causes significant amounts of distracting noise and vibration.
FIG. 2A shows another prior art approach that provides a quiet fan speed control 20. In this approach, two capacitors 24, 25 are coupled in series electrical connection between the AC power source 16 and the fan motor 18. Two controllably conductive switches 22, 23, for example, bidirectional semiconductor switches, such as triacs, are provided in series with each of the capacitors 24, 25. A control circuit 26 is operable to control the conduction state of the switches 21, 22, 23 in order to selectively switch one or both of the capacitors 24, 25 in series electrical connection with the fan motor. Accordingly, a voltage divider is formed between the capacitors 24, 25 and the fan motor 18. Different values of capacitance in series with the fan motor 18 produce different voltages across the fan motor, which induce different fan speeds. Typically, as the capacitance in series with the fan motor 18 decreases, the speed of the fan motor will also decrease.
By controlling the switches 22, 23 to selectively insert and remove the capacitors 24, 25 from the circuit, the control circuit 26 can provide a plurality of discrete fan speeds. If either of the switches 22, 23, or any combination of these switches, are conductive, the fan motor will operate at one of the discrete speeds depending upon the equivalent capacitance in series between the AC power source 16 and the fan motor 18. The control circuit 26 drives each triac that must be conductive for a select one of the discrete speeds into substantially full conduction, i.e., the triac conducts approximately the entire length of each half-cycle. Since the fan motor 18 has a large resistive component, the motor current through the fan motor leads the AC input voltage of the AC power source 16 (i.e., is out-of-phase with the AC input voltage) when one or more of the capacitors 24, 25 in coupled in series with the fan motor.
A bypass switch 21 is also controlled by the control circuit 26. When the bypass switch 21 is conductive, the full AC input voltage of the AC power source 16 is provided to the fan motor 18, which then operates at substantially full speed. Accordingly, with the circuit shown in FIG. 2A, as many as four different discrete speeds can be obtained (including the full speed). Additional capacitors and switches can be provided to obtain more discrete speed levels, but the circuitry becomes unnecessarily complex, large, and expensive as more components are added. An example of this type of speed control is shown in U.S. Pat. No. 4,992,709, issued Feb. 12, 1991, entitled SWITCHING CIRCUIT PROVIDING ADJUSTABLE CAPACITIVE SERIES VOLTAGE DROPPING CIRCUIT WITH A FRACTIONAL HORSEPOWER MOTOR, the entire disclosure of which is hereby incorporated by reference.
FIG. 2B shows waveforms of the line voltage 31A, motor voltage 311B, and motor current 31C for the prior art fan speed control 20 of FIG. 2A. As can be observed, the waveforms are fairly continuous and smooth, lacking the discontinuities of the system shown in FIG. 1A. Since the switches 21, 22, 23, are either on or off, and not operated according to the phase control technique of the fan speed control 10 of FIG. 1A, the waveforms do not exhibit discontinuities. Accordingly, minimal noise is generated in the fan motor when the fan speed control 20 is operating in a steady-state condition, i.e., at one of the discrete fan speeds.
However, the fan speed control 20 is susceptible to generating noise when the control circuit 26 changes the speed of the fan motor 18, i.e., when the control circuit changes the conduction state of the switches 22, 23. For example, consider the fan speed control 20 operating with the switch 22 conductive and the switch 23 non-conductive, such that only the capacitor 24 is coupled in series with the fan motor 18. The capacitor 24 will charge and discharge each line cycle in accordance with the AC line voltage provided by the AC power source 16. Assuming that the switch 23 has been non-conductive for a long time, the capacitor 25 will have a substantially low charge, i.e., only a small voltage will be developed across the capacitor 25. To change the speed of the fan motor 18, the control circuit 26 is operable to render the switch 23 conductive and keep the switch 22 conductive. If the control circuit 26 renders the switch 23 conductive when the voltage across the capacitor 24 is substantially different from the voltage across the capacitor 25, a large circulation current will be produced and will flow through both of the capacitors. This large current will cause the plates of the capacitors 24, 25 to contract, making an audible “clicking” noise, which can be annoying to a user of the fan speed control 20. Repetitive occurrences of such a large current can damage the capacitors and other electrical parts of the fan speed control 20, thereby decreasing the life of the fan speed control.
Some prior art fan speed controls have included discharge resistors, for example, resistors 27, 28 of the fan speed control 20 shown in FIG. 2A. The discharge resistors have small resistances and allow the capacitors 24, 25 to discharge quickly. Also, the fan speed control 20 may include limiting resistors 29, 30 to limit the peak discharge current. When changing speeds of the fan motor 18, the control circuit will cause the switches 22, 23 to be non-conductive for a predetermined period of time to allow the capacitors to discharge, before rendering one or more of the capacitors conductive. However, since a period of time is required to allow the capacitors 24, 25 to discharge, this method of control limits the speed at which the fan speed control 20 can change the speed of the fan motor 18. Further, the discharge resistors 27, 28 dissipate a large amount of power during normal operation of the fan speed control 20, thus requiring rather large and expensive resistors. Accordingly, there is a need for a quiet fan speed control that can quickly change the speed of the fan motor without generating excessive noise in the fan speed control and that does not generate excessive heat during normal operation.
Furthermore, the fan motor 18 often has trouble starting up when the fan motor is turned on to a very low speed from off. To overcome this problem in the prior art fan speed control 20, the control circuit 26 initially “kick starts” the fan motor 18 by driving the fan motor at the maximum speed possible, i.e., rendering the bypass switch 21 conductive, for a predetermined period of time. After this period of time, the fan motor 18 will be rotating with an acceptable amount of inertia and the control circuit 26 will then control the switches 24, 25 to switch the appropriate capacitance in series with the fan motor 18 to produce the desired lower speed. However, changing the speed of the fan motor 18 from the initial off speed to the full speed, and then back down to the desired low speed, can generate a large pulse of current through the fan motor, which can cause the fan motor to make an audible “clunking” noise. As previously mentioned, acoustic noises produced in the fan motor can be annoying and distracting to a user. Accordingly, there is a need for a quiet fan speed control that is able to start up a fan motor to a low speed without causing the fan motor to make excessive noise.